a) Field of the Invention
The present invention relates to cleaning a reaction chamber and its tubes of a semiconductor device manufacturing apparatus by using ClF.sub.3.
b) Description of the Related Art
Thin film forming techniques such as chemical vapor deposition (CVD) and physical vapor deposition (PVD) are incorporated in semiconductor device manufacturing processes. As a thin film is formed by such a technique on a semiconductor substrate placed in a reaction chamber, a thin film is also grown on the inner wall of the reaction chamber and the surfaces of jigs. As the attached thin film becomes thick, for example, to several tens .mu.m, a fraction of the thin film peels off as particles. Particles may cause a low manufacturing yield. It is therefore necessary for a thin film forming apparatus to be subjected to routine cleaning for removing attached films.
Cleaning is broadly classified into wet cleaning and dry cleaning. For example, in wet-cleaning silicon based thin films, a reaction tube and jigs are dipped in a mixed solution of hydrofluoric acid and nitric acid. During the whole cleaning process, a thin film forming apparatus is cooled to the room temperature and disassembled into a reaction tube and jigs which are then wet-cleaned and reassembled into a thin film forming apparatus which is again heated. The most common current process takes at least 16 to 24 hours.
Dry cleaning is performed without disassembling a thin film forming apparatus into a reaction tube and jigs and without cooling the apparatus. Therefore, dry cleaning is called in situ cleaning. The whole cleaning process can be simplified considerably, improving apparatus uptime and throughput. The number of heating/cooling cycles is reduced, prolonging the service life time of a reaction tube. Dry cleaning is particularly useful for a reaction tube such as a quartz reaction tube with a thin film of a different thermal expansion coefficient being attached thereto, because if the tube is cooled, thermal stress causes the generation of cracks on the inner surface of the quartz tube. The tube with more cracks should be discarded because it is mechanically weaker.
Known dry cleaning gas is a gas containing fluorine such as NF.sub.3 and ClF.sub.3. NF.sub.3 gas is used in the plasma assisted etch processing whereas ClF.sub.3 gas is used as it is. As a cleaning gas for a thin film forming apparatus without a plasma formation device, ClF.sub.3 is therefore more attractive.
Exhaust gases, produced during ClF.sub.3 gas cleaning process, are quite toxic and should be removed quickly. It is also desired to prevent metal and sealing materials from being corroded by cleaning gas. From these reasons, ClF.sub.3 gas is used by diluting it by a large amount of inert gas such as N.sub.2 and Ar. If an adsorption type toxic waste remover or another type toxic waste remover such as a scrubber is used, ClF.sub.3 gas diluted to 5% or smaller is generally introduced into a reaction chamber. In this case, even if an exothermic reaction occurs in a toxic waste remover, the temperature of the remover can be suppressed to be about 40.degree. C. or lower, ensuring the safety of operation.
A method of cleaning a semiconductor device manufacturing apparatus by using ClF.sub.3 is disclosed in Japanese Patent Laid-open Publication No. 5-226270.
FIG. 8A is a schematic cross sectional diagram of a CVD system for forming a film of tungsten (W) or tungsten silicide (WSi) reproduced from the Publication No. 5-226270. A susceptor 101 for supporting a substrate is disposed at the center of the bottom of a reaction chamber 100 made of aluminum or the like. A substrate 102 is placed on the susceptor 101 to deposit a film on the substrate 102.
The tip of a gas pipe 104 is guided into the chamber 100 above the susceptor 101. A perforated plate 103 is mounted surrounding the tip of the gas pipe 104. Source gas or cleaning gas blown out of the tip of the gas pipe 104 is introduced into the reaction chamber 100 via holes of the perforated plate 103.
A drain pipe 105 is coupled to the bottom of the reaction chamber 100, and is communicating with a vacuum pump (not shown). The source gas or cleaning gas in the reaction chamber 100 is exhausted via the pipe 105.
FIG. 8B is a schematic cross sectional view of a conventional lateral type CVD system. Flanges 111 and 112 are mounted on opposite ends of a reaction tube 110 made of quartz or the like. A gas pipe 114 is coupled to the side wall of the flange 111 so that cleaning gas containing C1F.sub.3 can be introduced. The center hole of the flange 111 is sealed by a cap 113.
A drain pipe 115 is coupled to the other flange 112 so that gas in the reaction tube 110 can be exhausted. A heater 116 is installed surrounding the reaction tube 110.
Cleaning gas introduced into the reaction chamber 100 shown in FIG. 8A or into the reaction tube 110 etches a film attached to the inner wall when films have been deposited on substrates, the attached film containing tungsten, tungsten silicide, polycrystalline, silicon, or other substances. The cleaning gas after etching is exhausted to the outside of the drain pipe 105 or 115. The inner wall of the reaction chamber 100 or the reaction tube 110 can be cleaned in the above manner.
With the conventional cleaning method described above, cleaning gas reacted with the attached film at the upstream region flows downstream. Therefore, the concentration of ClF.sub.3 is high at the upstream region, and low at the downstream region. The etching rate at the inner wall of the chamber or tube is therefore high at the upstream region.
Although ClF.sub.3 etches polycrystalline silicon attached to the inner wall of the chamber or tube, it also etches a small amount of quartz itself. Therefore, if a reaction chamber made of quartz is cleaned, the cleaned inner wall, e.g. at the upstream gas region is likely to be damaged.
The inner wall of the drain pipe is cleaned slightly by residual ClF.sub.3 in the exhausted cleaning gas, but its cleaning effect is relatively small. A gas supply pipe used only for supplying gas for the deposition of a film on a substrate is not cleaned at all.
In order to improve a throughput of a manufacturing apparatus, it is necessary to increase an etching rate and reduce a cleaning time. An increase of an etching rate requires an increased flow rate of ClF.sub.3 which raises cost.
Furthermore, since the thickness of a polycrystalline silicon film attached to the inner wall of a chamber is not uniform, the chamber inner wall is exposed at the thin film area after the film is fully etched. The exposed area is overetched by ClF.sub.3 and damaged. As a result, as the cleaning is repeated, the transparency of the quartz chamber is lost (devitrlficated).